Question 998947
.
Let  j  be the number of minutes for John to clean the tank.


Let  m  be the number of minutes for Mark to clean the tank.


Then John will clean  {{{1/j}}}  part of the tank volume per minute.

Correspondingly,  Mark will clean  {{{1/m}}}  part of the tank volume per minute.


Working together,  John and Mark will clean  {{{1/30}}}  part of the tank volume per minute.  It means that


{{{1/j}}} + {{{1/m}}} = {{{1/30}}}.       (1)


From the other part of the condition,  Mark is cleaning  {{{1/50}}}  part of the tank volume per minute working alone.  It means that


{{{1/m}}} = {{{1/50}}}.             (2)


Substitute  (2)  into  (1).  You will get


{{{1/j}}} + {{{1/50}}} = {{{1/30}}}.


Hence,


{{{1/j}}} = {{{1/30}}} - {{{1/50}}} = {{{(50-30)/1500}}} = {{{20/1500}}} = {{{2/150}}} = {{{1/75}}}.


Thus  j = {{{75/1}}} = 75 minutes.


<U>Answer</U>. &nbsp;It will take &nbsp;75 &nbsp;minutes for Mark to clean the tank working alone.